Generate number series

public string incrementString(string input)
        {
                           
            char[] c = input.ToCharArray();
            string output;
            string sDigitValue = new string(c);
            string sDefineValue = sDigitValue.Substring(1, 3);
            string sFirstChar = sDigitValue.Substring(0, 1);

            int i = c.Length - 1;
            
            bool overflow = false;
            bool nextdigit = false;
            do
            {
                if (i < 0)
                {
                    nextdigit = true;
                    overflow = false;
                }
                else if (sDefineValue.ToString().ToUpper() == "9ZZ")
                {
                    lblMessage.Text = "Code Limit Exceeds...Consult System Admin.";
                    break;
                }
                else
                {
                    int isIntValue;
                    bool isNumeric = int.TryParse(sDefineValue, out isIntValue);

                    if (isNumeric == true)
                    {
                        if (Convert.ToInt32(sDefineValue) == 999)
                        {
                            sDefineValue = "9A0";
                            output = sFirstChar + sDefineValue;
                            return output;
                        }
                        
                        else
                        {   
                            isIntValue = Convert.ToInt32(sDefineValue) + 1;
                            output = sFirstChar + isIntValue.ToString("000");
                            return output;

                        }
                    }

                    else
                    {
                        c[i]++;

                        if (c[i] == ':')
                        {
                            c[i] = 'A';     // jump from ":" to "A" in ascii table
                        }

                        if (c[i] > 'Z')
                        {
                            // roll over to next digit
                            c[i] = '0';
                            overflow = true;
                            i--;
                        }
                        else
                        {
                            overflow = false;
                        }
                    }

                }
            }

            while (overflow);
            if (nextdigit)
            {
                output = String.Concat("1", new string(c));
            }
            else
            {
                output = new string(c);
            }
            return output;
        }



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